Prior to the advent of the industrial age, standardization of tools and mechanical components was unknown. Prior to this standardization, workers were forced to spend considerable time selecting closely fitting components from a wide range of such components, having a practically infinite variation in size. Standardization of tools and gauges for use with such fasteners, was of course impossible.
With the automation of many manufacturing and assembly processes, the standardization of various components (e.g., nuts and bolts) as well as the tools used therewith, was developed in order to speed the process. The result is that the mechanic or craftsman may select an appropriately sized fastener and mating tool (wrench, etc.) for use with that fastener, relatively rapidly from a selection of a relatively few finite sizes.
While an experienced mechanic is generally able to judge the size of a fastener reasonably accurately by eye, and thus accurately select the appropriate wrench or socket for use with the fastener, this skill can be a difficult one to acquire and is not a perfect art, even for experienced workers. This often leads to some lost time, as the mechanic looks at e.g. a nine millimeter bolt head and erroneously picks up a ten millimeter socket to apply to the bolt. Each time such a minor "cut and try" error is made, the lost time may be practically insignificant, but the time lost to such minor errors can add up significantly in a major job.
This is all the more so, when the work project is remotely located from the mechanic's tools, resulting in the mechanic returning to his/her toolbox, selecting a wrench or socket, returning to the job site, finding the tool is the wrong size, returning to the tool box to try another size tool, etc. Such travels are exasperating to say the least, when a mechanic may be working under a vehicle, and the toolbox is located at the opposite end of the shop. As nuts and bolts are not marked with dimensions, the only solution is to try different tools until the proper tool is found, unless the fastener is accurately measured first.
Few mechanics take the time to measure such fasteners before selecting a tool, generally because the act of measuring the fastener is as inconvenient as erroneously selecting the wrong size tool and having to make a second selection. A standard linear ruler can be difficult to use due to parallax, a recessed fastener which is difficult to access, etc. While some have used calipers for such measurements, most calipers are vernier, and provide much finer measurement than needed for such a task. They are also delicate and prone to damage in such a harsh environment.
Accordingly, a need will be seen for a wrench selector providing for the quick and precise determination of the exact size of a nut or bolt comprising one of a family of standard sizes of fasteners. The selector must provide a positive and accurate indication of the exact size of the standard fastener, without requiring interpolation or other additional steps in the determination of the measurement or dimension. The device should provide for the determination of the size of fasteners of the two most common standards used, i.e., the English system of fractional inch and inch dimensions, and the metric system using millimeters. Finally, the device must be sufficiently small that it may easily be carried on the person (shirt pocket, etc.) in order to be available at all times so the user will not be forced to travel back to a tool bench or box to retrieve the device, as with an improperly sized tool.